This application relates to a system for separating metallic contaminants from an ore slurry.
The mining industry utilizes various devices to separate valuable minerals from host contaminants after extraction from the earth. Initially, the ore preparation procedure involves crushing the run-of-mine rock from several feet in size down to approximately 1 to 3 inches. This preliminary crushing step is followed by one or more stages of grinding to reduce the ore to an average size of less than 1 millimeter. These latter grinding steps typically use large rotating cylindrical mills containing a charge of spherical steel balls that are used as a grinding media. The balls are in a constant tumbling motion due to the rotation of the mill. The ore is fed into one end of the mill, progresses through the grinding chamber, and is discharged from the opposite end. As the ore progresses through the mill, the grinding media impacts the material, resulting in fracture and breakage of the individual pieces into smaller and smaller particles.
The tumbling motion of the balls can also result in fracture of the balls themselves. Additionally, mechanical abrasion will wear the ball surface causing a reduction in size of the grinding media. The net results of this process are the generation of various shapes of steel which are significantly smaller than the original spherical balls, and the further contamination of the ore with metal fragments from the balls. Depending on the mill design, these fragments will discharge with the ground-up ore particles and flow to downstream equipment.
The ball fragments cause two distinct problems in ore processing facilities. The first is wear on subsequent equipment. Grinding is typically a wet process and the ore/water slurry is pumped between various unit operations. The metallic fragments cause significant wear on pumps, piping, and other downstream equipment. The costs associated with maintenance downtime and equipment repair/replacement can be substantial. Second, the ball fragments reduce the efficiency of the grinding mill itself. In most grinding operations, the balls and ball fragments that discharge from the grinding mill are returned to the grinding circuit with new ore feed. As a result, a substantial build-up of fragments can occur in the grinding mill occupying volume that would otherwise be filled by mineral slurry. This loss in active mill volume can decrease the mill capacity by a substantial amount. Furthermore, the small mass of the fragments does not provide a sufficient impact force to effectively fracture the mineral particles in the grinding mill.
U.S. Pat. No. 2,332,701 recognizes the problem represented by worn and fragmented grinding media in ball mills, and discloses a ball mill that continuously discharges grinding media with the ground material and returns to the feed end of the mill only that portion of the grinding media which is in good condition and of the correct size. A trommel screen sorts the output and an elevator conducts the useful grinding media back to the feed end of the mill. However, the trommel screen would not be able to separate out ball fragments smaller than the holes in the trommel screen, and the trommel screen cannot be made too fine or else it would become clogged by the ore slurry.
Recognizing the limitations of physical screening/separation, methods have been developed in the art for separating ferrous material from non-ferrous wet or dry materials using magnetic drum separators, which are well-known in the art. One of such methods is exemplified by U.S. Pat. No. 6,149,014, which discloses a top-fed, wet magnetic drum separator to remove metallic contaminants from the discharge slurry of an operating grinding mill. In the invention disclosed in U.S. Pat. No. 6,149,014, slurry discharging from the grinding mill enters an enclosed feed box located on top of the separator. The enclosed feed box provides a physical velocity break, minimizes turbulence, and then spreads the mineral slurry on the drum surface in a contained manner by means of a flexible xe2x80x9cfeed introducerxe2x80x9d that extends from the feed box to actually engage the drum surface; the velocity break section is expressly required to be large enough to provide overflow of feed material. The feed box and drum surface are rubber-lined to minimize wear. Barrier walls attached to and rotatable with the drum surface contain the flow of slurry in conjunction with a sealed, curved, stationary cover located over the barrier walls to force slurry around the curvature of the drum, thereby maintaining the slurry in sufficient contact with the drum to enable magnetic separation to take place. Without the cover to contain it, the slurry would tend to be ejected from the enclosed feed box at a relatively high velocity with little contact with the drum surface and little opportunity for magnetic separation to take place. Once the cover forces the slurry into contact with the drum, a fixed magnet assembly arranged in an arc within the drum from approximately 26 to 218.5 degrees (pole center-to-center) starts attracting ferrous material within that slurry so as to begin the process of magnetic separation.
Further, U.S. Pat. No. 6,149,014 teaches the use of cleats on the drum surface to ensure transfer of metal fragments around the drum surface and discharge beyond the last pole. A partitioned product hopper is located around the lower portion of the separator, configured with a physical splitter positioned before the last pole to physically partition metallic fragments from the slurry. A drum spray bar is located beyond the last pole to remove solids that continue to adhere to the drum surface after the slurry and any metal fragments have fallen off, to prepare and clean the drum surface to receive further slurry from the feed box for separation.
The method and apparatus used in the U.S. Pat. No. 6,149,014 invention have a number of disadvantages:
(1) Expressly required by U.S. Pat. No. 6,149,014 is an enclosed discharge slurry feed box large enough to provide for overflow capacity and a velocity break section, resulting in unnecessary size and capacity to hold the tremendously large volumes of discharge slurry that are periodically generated during the grinding mill production cycle. The overflow capacity necessarily requires the feed box capacity be sufficiently large to accommodate maximum volume, which is wasteful, and thereby underutilizes that capacity at other times, necessarily making the U.S. Pat. No. 6,149,014 invention excessively large when space constraints are often a prevalent concern in or around grinding mills. This excess volume capacity and size, renders the invention less useful in many field applications without expensive retrofits.
(2) In the U.S. Pat. No. 6,149,014 invention, all of the discharge slurry is forced through an enclosed feed box and directly into a very confined conduit made up of the outer surface of the drum shell, the barrier walls, and the stationary cover. Therefore, the velocity break section in the feed box is essential to prevent the slurry from overwhelming the magnetic drum, there being no other means provided for this purpose. This necessarily increases the amount of wear and abrasion in the feed box, on the outer surface of the drum shell, on the barrier walls, and on the stationary cover used to guide the slurry around the curvature of the outer surface of the drum shell.
(3) In addition to the wear and abrasion created by the full volume and rate of flow emanating from the discharge slurry, the invention is a fully enclosed system that can generate head pressure within the confines of the internal vessels and greatly increases the risk of wear, abrasion, and internal damage.
(4) The U.S. Pat. No. 6,149,014 invention is fully enclosed, making it difficult to maintain and repair. Although it is internally lined with rubber to resist wear and abrasion, standard rubber lining is not designed for quick replacement and is not easily accessible without substantially dismantling the device. This is a costly and time-consuming problem, especially in the milling industry where long shutdowns for repair or replacement are extraordinarily expensive for milling operations.
(5) The system disclosed in U.S. Pat. No. 6,149,014 would not work if it were not fully enclosed. The stationary cover is required to contain the slurry against the outer surface of the drum; otherwise, any slurry being ejected from the enclosed feed box at high velocity or volume would not be kept within sufficient contact with the drum to enable magnetic separation to take place.
This invention provides a magnetic separator system comprising an open trunnion discharge feed chute and an open magnetic drum separator for separating ferrous material from a discharge slurry. In particular, a magnetic separation system according to the present invention comprises:
(1) a feed chute for receiving discharge slurry, the feed chute comprising walls and a feed chute floor, the feed chute floor having an adjustable feed chute floor portion for controlling the release of discharge slurry from the feed chute; and
(2) a magnetic drum separator comprising a drum rotatable about an axis below the feed chute, the drum having a generally cylindrical outer surface and having a fixed magnet assembly within that rotating outer surface.
By adjusting the adjustable chute floor portion, the user can control where discharge slurry released from the feed chute contacts the outer surface of the drum. Therefore, simply by adjusting the adjustable chute floor portion of the feed chute, the flow of discharge slurry can be made to contact the outer surface of the drum only at the optimal point of contact, and that optimal point of contact can be maintained in this same way, thereby compensating for variations in the flow of discharge slurry. This ability to control with precision the flow of discharge slurry onto the outer surface of the drum permits the system to use an open construction, rather than an enclosed construction as required in prior art system to force discharge slurry into contact with the drum. This open construction not only reduces wear and tear given that discharge slurry is no longer being forced through physical enclosures, but the open construction also provides greater ease of accessibility for repair and replacement where needed. Further, the open feed chute eliminates the need for an unnecessarily large feed box with a separate overflow section, so the magnetic separation system of this invention can be of a more compact construction. Also, a physical velocity break is no longer necessary since this invention already compensates for variations in flow of discharge slurry.
In operation, a method of removing ferrous material from a discharge slurry flow according to this invention comprises:
(1) discharging discharge slurry into the feed chute described above;
(2) releasing the discharge slurry from the feed chute onto the outer surface of the rotating drum described above;
(3) rotating said outer surface of the drum in the direction of the flow of said discharge slurry;
(4) adjusting the adjustable feed chute floor portion so as to maintain, at a predetermined point, the area at which discharge slurry released from the feed chute contacts the outer surface of the drum;
(5) separating ferrous material from the discharge slurry by magnetically attracting ferrous material to the drum while allowing non-ferrous discharge slurry to flow past and off the drum; and
(6) conveying ferrous material away from the non-ferrous discharge slurry and then discharging the ferrous material from the drum at a different location.
The radial field of magnetic attraction provided by the fixed magnet assembly preferably extends not just around the circumference of the drum, but also into the feed chute over the adjustable feed chute floor portion. This enables magnetic separation to take place in two phases: the first phase taking place while the discharge slurry is still flowing in the feed chute, and the second phase taking place after the discharge slurry falls into contact with the outer surface of the drum.
Preferably, the drum is covered by a replaceable protective lining constructed in segments that are easily installed and removed, but which overlap and interlock with one another to form a sealed protective lining. The segments are all preferably constructed of a non-ferrous elastomeric material. The outer surface of the drum also preferably has a plurality of cleats to assist the transfer of ferrous material around that outer surface. The cleats are preferably strengthened by a metallic bar through the length of each cleat.
An electronically-controlled adjuster can be programmed to automatically adjust the adjustable feed chute floor portion in response to variations in flow of discharge slurry. Further, a variable speed motor can be used to rotate the drum, varying the speed of rotation in response to variations in the flow of discharge slurry. This would not have been effective in respect of prior art enclosed systems, since increasing the speed of rotation would simply create more pressure within the physical enclosures and cause more wear and abrasion.